Taylor Swift treated fans to a special surprise guest on Saturday night during her Red Tour stop in Foxborough, Massachusetts, bringing Carly Simon out to join her in singing Simon's 1972 hit "You're So Vain."

"Before I bring out my special guest, I have this question that I've always, always had," Swift told the Gillette Stadium audience while introducing the famously speculated-upon song. "I think a lot of you have probably had this question too, always wondered what the answer is. Who is this song, 'You're So Vain' by Carly Simon, written about?" (Guesses have ranged from David Geffen to Cat Stevens and Warren Beatty.)

After the show, Swift's excitement bubbled over onto Twitter. "Got to sing 'You're So Vain' with Carly Simon at Gillette Stadium tonight!!!!!" she wrote. "Unreal. I could not possibly look up to her more."